This invention relates to a paper material for photosensitive materials, such as light-shielding paper wound together with a rolled photosensitive material, a support used for a photographic printing paper or an instant film, or the like, and a method of producing the same.
Conventional light-shielding papers wound together with rolled photosensitive material are light-shielding black paper or a paper support on which an EVA resin solution containing more than 5 wt. % of carbon black dissolved in a solvent such as toluene is coated by gravure coating.
The light-shielding paper produced by coating an EVA resin solution onto a paper support is excellent in flexibility and conductivity, and the adhesive force between the EVA resin layer and the paper support is sufficient. However, it is necessary to evaporate the solvent such as toluene, and accordingly explosionproof equipment which is expensive is necessary. Moreover, unless the coating is less than 50 m/min., membrane rupture, pinholes or craters which render moistureproofness and light-shielding incomplete occur on the coating layer due to air contained in the paper support. Thus, the above light-shielding paper is inferior with respect to its low manufacturing rate and manufacturing cost. Furthermore, the residual solvent therein induces blocking between not only the photosensitive layer and the light-shielding paper of the rolled photosensitive material but also between the light-shielding papers in a wound state, with fogging, sensitivity deviation or the like of the photosensitive material, lump troubles caused by aggregation of carbon black occurring during the long coating process.
The applicant has investigated in order to resolve the above problems, and proposed a light-shielding paper formed by extrusion-laminating a polyethylene resin layer composed of a high density polyethylene resin and a low density polyethylene resin blended with carbon black onto a paper support (Japanese Patent KOKAI No. 60-35728).
Recently, a water-resistant support formed of a paper support having both sides coated by a polyolefin resin has gradually been utilized as the support for photographic printing paper instead of baryta paper according to the speeding up of development. The support for photographic printing paper is provided with a transparent or translucent polyolefin resin extrusion laminating layer on one side and with an opaque polyolefin resin extrusion laminating layer containing a white pigment which is light-reflective light-shielding material, such as titanium dioxide, on the other side. A photoemulsion layer is provided on the opaque polyolefin resin extrusion laminating layer to form photographic printing paper. Negative images are printed on the photographic printing paper, and then developed to obtain the printing paper having positive images. The printing paper is then cut into a prescribed size by a cutter or the like.
Shear force is added to the printing paper produced from the above support having the polyolefin resin extrusion laminating layer between the upper and lower blades of the cutter during cutting, and at that time, the printing papers have mostly not a simple cut end but an elongated one which degrades the appearance of the printing paper resulting in a reduction of its commercial value.
A method of using a low molecular weight polyethylene resin for the polyolefin resin extrusion laminating layer is proposed as a means to improve the cutability (Japanese Patent KOKOKU No. 58-41500).
However, the printing paper provided with the polyolefin resin extrusion laminating layer formed of the low molecular weight polyethylene resin tends to be damaged by contacting a metal portion or the like during traveling in a printer, a developing apparatus or the like, and moreover, it is inferior in resistance to curling.
Heretofore, various techniques have been proposed, for example, the method of using a high density polyethylene resin having specific molecular structure and properties (Japanese Patent KOKAI No. 58-95731), the method of using a polyethylene resin blend wherein a high density polyethylene resin having specific properties and a low density polyethylene resin having a specific properties are blended at a specific ratio (Japanese Patent KOKAI Nos. 60-150049, 63-237055).
The above conventional light-shielding paper formed of a paper support on which a polyethylene resin layer is laminated by extrusion laminating can be produced inexpensively because it does not require expensive explosionproof equipment, and does not induce fogging or the like on photosensitive materials.
However, when more than 5 wt. % of carbon black is blended in order to improve light-shielding and conductivity, adhesion between the polyethylene resin layer and the paper support becomes inferior. Moreover, foaming, film rupture and pinholes occur by the influence of moisture adsorbed on the carbon black. In the case that the thickness of the laminating layer is less than 40 .mu.m, film rupture and pinholes are particularly liable to occur. Furthermore, when the paper support has a great surface irregularity, a high moisture content or a small density, film rupture, pinholes or inferior adhesion are liable to occur by the influence of air or water vapor.
The aforementioned conventional support for photographic printing paper is excellent in cutting properties and resistance to abrasion due to the process of high density polyethylene resin. However, it is inferior in insufficient adhesive strength, great neck-in and has insufficient resistance to curling. Particularly, in the case of blending more than 3 wt. % of white pigment for the purpose of the improvement in shielding ability, particularly more than 5 wt. % for the purpose of the improvement in sharpness, there are problems of insufficient adhesive force and neck-in.